A main striving force in the development of wireless/cellular communication networks and systems is to provide, apart from many other aspects, increased coverage or support of higher data rate, or a combination of both. At the same time, the cost aspect of building and maintaining the system is of great importance and is expected to become even more so in the future. As data rates and/or communication distances are increased, the problem of increased battery consumption is another area of concern.
Until recently the main topology of wireless networks has been fairly unchanged, including the three existing generations of cellular networks. The topology characterized by the cellular architecture with the fixed radio base stations and the mobile stations as the transmitting and receiving entities in the networks, wherein a communication typically only involves these two entities. An alternative approach to networks are exemplified by the well-known multihop networks, wherein typically, in a wireless scenario, a communication involves a plurality of transmitting and receiving entities in a relaying configuration. Such systems offer possibilities of significantly reduced path loss between communicating (relay) entities, which may benefit the end-to-end (ETE) users.
Attention has recently been given to another type of topology that has many features and advantages in common with the multihop networks but is limited to only two (or a few) hop relaying. In contrast to multihop networks, aforementioned topology exploits aspects of parallelism and also adopts themes from advanced antenna systems. These networks, utilizing the new type of topology, have cooperation among multiple stations as a common denominator. In recent research literature, it goes under several names, such as cooperative relying, cooperative diversity, cooperative coding, virtual antenna arrays, etc. In the present application the terms “cooperative relaying” and “cooperative schemes/methods” is meant to encompass all systems and networks utilizing cooperation among multiple stations and the scheme/methods used in the systems, respectively. A comprehensive overview of cooperative communication schemes are given in [1]. Various formats of a relayed signal may be deployed. A signal may be decoded, re-modulated and forwarded, or alternatively simply amplified and forwarded. The former is known as decode-and-forward or regenerative relaying, whereas the latter is known as amplify-and-forward, or non-regenerative relaying. Both regenerative and non-regenerative relaying is well known, e.g. by traditional multihopping and repeater solutions respectively. Various aspects of the two approaches are addressed in [2].
The general benefits of cooperative relaying in wireless communication can be summarized as higher data rates, reduced outage (due to different forms of diversity), increased battery life, extended coverage (e.g. for cellular).
Various schemes and topologies utilizing cooperative relaying has been suggested, as theoretical models within the area of information theory, as suggestions for actual networks and in a few cases as laboratory test systems, for example. Examples are found in [1] pages 37-39, 41-44. The various cooperation schemes may be divided based on which entities have data to send, to whom and who cooperates. In FIGS. 1a-f (prior art) different topologies are schematically illustrated, showing where traffic is generated, who is the receiver and the path for radio transmissions.
The classical relay channel, illustrated in FIG. 1a, consists of a source that wishes to communicate with a destination through the use of relays. The relay receives the signal transmitted by the source through a noisy channel, process it and forwards it to the destination. The destination observes a superposition of the source and the relay transmission. The relay does not have any information to send; hence the goal of the relay is to maximize the total rate of information flow from the source to the destination. The classical relay channel has been studied in [1], [7] and in [3] where receiver diversity was incorporated in the latter. The classical relay channel, in its three-station from, does not exploit multiple relay stations at all, and hence does not provide the advantages stated above.
A more promising approach, parallel relay channel, is schematically illustrated in FIG. 1b, wherein a wireless systems employing repeaters (such as cellular basestation with supporting repeaters) with overlapping coverage, a receiver may benefit of using super-positioned signals received from multiple repeaters. This is something that happens automatically in systems when repeaters are located closely. Recently, information theoretical studies have addressed this case. A particular case of interest is by Schein, [4] and [5]. Schein has performed information theoretical study on a cooperation-oriented network with four nodes, i.e. with one transmitter, one receiver and only two intermediately relays. A real valued channel with propagation loss equal to one is investigated. Each relay employs non-regenerative relaying, i.e. pure amplification. Thanks to the simplistic assumption of real valued propagation loss, the signals add coherently at the receiver antenna. Under individual relay power constraints, Schein also indicates that amplification factors can be selected to maximize receiver SNR, though does not derive the explicit expression for the amplification factors. One of the stations sends with its maximum power, whereas the other sends with some other but smaller power. The shortcoming of Schein's schemes is that it is; only an information theoretical analysis, limited to only two relay stations, derived in a real valued channel with gain one (hence neglecting fundamental and realistic propagation assumptions), lacks the means and mechanisms to make the method practically feasible. For example, protocols, power control and RRM mechanisms, complexity and overhead issues are not addressed at all. With respect to only addressing only two relay stations, the significantly higher antenna gains and diversity benefits, as would result for larger number of relays, are neither considered not exploited.
The concept of multiple-access Channel with Relaying (a.k.a. as Multiple access channels with generalized feedback) has been investigated by several researchers lately and is schematically illustrated in FIG. 1c. The concept involves that two users cooperate, i.e. exchange the information each wants to transmit, and subsequently each user sends not just its own information but also the other users information to one receiver. The benefit in doing so is that corporation provides diversity gain. There are essentially two schemes that have been investigated; cooperative diversity and coded cooperative diversity. Studies are reported in [1], foe example. With respect to diversity, various forms has been suggested, such as Alamouti diversity, receiver diversity, coherent combining based diversity. Typically the investigated schemes and topologies rely on decoding data prior to transmission. This further means that stations has to be closely located to cooperate, and therefore exclude cooperation with more distant relays, as well as the large number of potential relays if a large scale group could be formed. An additional shortcoming of those schemes is that is fairly unlikely having closely located and concurrently transmitting stations. These shortcomings indicates that the investigated topology are of less practical interest. The broadcast channel with relaying, illustrated in FIG. 1d, is essentially the reverse of the topology depicted in FIG. 1c, and therefore shares the same severe shortcomings.
A further extension of the topology depicted in FIG. 1c, wherein two receivers are considered. This has e.g. been studied in [8] and [1] but without cooperation between the receivers, and hence not exploiting the possibilities possibly afforded by cooperative relaying.
Another reported topology, schematically illustrated in FIG. 1f, is sometimes referred to as Virtual Antenna Array Channel, and described in for example [9]. In this concept, (significant) bandwidth expansion between a communicating station and adjacent relay nodes is assumed, and hence non-interfering signals can be transferred over orthogonal resources that allows for phase and amplitude information to be retained. With this architecture, MIMO (Multiple Input Multiple Output) communication (but also other space-time coding methods) is enabled with a single antenna receiver. The topology may equivalently be used for transmission. A general assumption is that relay stations are close to the receiver (or transmitter). This limits the probability to find a relay as well as the total number of possible relays that may be used. A significant practical limitation is that very large bandwidth expansion is needed to relay signals over non-interfering channels to the receiver for processing.
Cooperative relaying has some superficial similarities to the Transmit diversity concept in (a.k.a. Transmit diversity with Rich Feedback, TDRF), as depicted in [10] and is schematically illustrated in FIG. 1g. Essential to the concept is that a transmitter with fixed located antennas, e.g. at a basestation in a cellular system, finds out the channel parameters (allowing for fading effects and random phase) from each antenna element to the receiver antenna and uses this information to ensure that a (noise free) signal, after weighting and phase adjustment in the transmitter, is sent and adds coherently at the receiver antenna thereby maximizing the signal to noise ratio. While transmit diversity, with perfectly known channel and implemented in a fixed basestation, provides significant performance benefits, it also exist practical limitations in terms of the number of antenna elements that can be implemented in one device or at one antenna site. Hence, there is a limit in the degree of performance gain that can be obtained. A disadvantage for basestation oriented transmit diversity is also that large objects between transmitter and receiver incur high path loss.
A significant shortcoming of the above discussed prior art is that they only enable and exploits a few, typically only two, stations to cooperate. The, in the art, proposed topologies and methods do not take full advantage of the anticipated advantages of a network with cooperative relaying that comprises a larger number of relaying stations. In particular, the proposed topologies and methods do not provide the necessary means to scale up the networks. One problem is that the control overhead between involved stations may become a large problem when many nodes are involved. In the worst case, more protocol overhead is sent than data traffic. A further problem is that the algorithms and processing means are not designed to manage large number of relay nodes, or increasing number of relay nodes i.e. the scaling issues in large cooperative relay networks has not been properly addressed.